: Despite the widespread use if prophylactic immune globulin, HDN, secondary to maternal sensitization to the RhD antigen, continues to complicate 6 in 1,000 births in the United States. In severe cases, the intrauterine transfusion of red cells to the fetus can be life-saving. However, despite this technical advancement, 15 to 25% of fetuses still succumb to their anemia. The long-term objective of this investigation is to develop a unique immunologic approach to the treatment of red cell alloimmunization in pregnancy, thereby negating the need for invasive fetal therapy. Such an advance, will allow couples plagued by this disease to more likely achieve a successful outcome to their pregnancy. The first aim of this investigation is to establish an animal model for hemolytic disease of the newborn. Female rabbits will be sensitized to incompatible red cells then bred with a buck with a compatible red cell type producing a control litter. Fetal blood sampling using ultrasound guidance will be used to assess levels of fetal hemoglobin. Neonatal hemoglobins and assessment for hepatosplenomegaly will also be undertaken. The does will be bred a second time with a red cell incompatible buck producing an affected anemic litter. Fetal and neonatal assessments will be repeated. The second aim of the study will be to alloimmunize does to paternal leukocyte antigens. Bucks that are incompatible with specific does at multiple rabbit leukocyte antigen (RLA) loci will be given a series of injections with granulocyte stimulating factor to increase the circulating white blood cell pool. Multiple immunizations of the does will then be undertaken using white cells from these bucks; anti-RLA titers will de assessed using flow cytometry. The third aim of the study will be to prove that immunization to paternal RLA antigens will decrease the severity of the HDN as compared to the second, incompatible breeding. A third breeding will be undertaken with the buck that is incompatible for both red cell and RLA antigens. Fetal and neonatal assessments will be repeated as in the two prior breeding. The fourth aim of the investigation will be to determine if the antibodies to the RLA-A or RLA-D locus are responsible for the protective effect for HDN of paternal anti-RLA antibodies. Specific does will be matched to bucks for either their RLA-A or RLA-D loci before their initial alloimmunization to white blood cells. Incompatible breeding will then be undertaken. The fifth aim of the study will be to investigate the in vitro protective effects of anti-RLA antibodies. A monocyte monolayer assay will be performed using the spleens and sensitized red cells of the pups from the three serial breedings to determine the level of phagocytosis of the fetal reticuloendothelial system.